Conventional power distribution systems for ships or other vehicles typically employ one or more prime movers (such as a steam turbine or diesel engine used to drive an electric generator or alternator) to provide a single frequency power supply current on a bus for powering and controlling variable speed motors to operate a propeller in association with supplying auxiliary power to other ship loads (e.g., a single frequency A/C input pump, blower, lamp, or other electrical A/C and D/C loads). In certain conventional power distribution systems for ships or other vehicles, the auxiliary loads and main propulsion loads may be driven by separate prime movers or generators via separate power distribution busses. Thus, in such conventional power distribution systems, the auxiliary loads and main propulsion loads are not electrically connected to each other via the same power distribution bus.
A conventional electrical power distribution system 10 for a ship, as shown in FIG. 1, typically employs multiple generators 12a-12d to source a single frequency, single voltage power distribution bus 14. To operate the ship's propeller 16, two variable, high speed motors 18a-18b capable of rotating a rotor shaft 20 within 0-600 RPM are typically coupled to the propeller 16 through a speed reducing gear box 22. Since the conventional power distribution bus 14 only supplies a single frequency and voltage, two conventional high power (e.g., 1000 kW output or more), variable frequency drives (VFDs) 24a-24b are required to supply the variable frequency input needed to operate the two high power, variable speed motors 18a-18b at a range of speeds. Accordingly, as shown in FIG. 1, VFDs 24a-24b are commonly coupled between the single frequency, single voltage power distribution bus 14 (usually via a respective transformer 26a-26b) and the variable, high speed motors 18a-18b. Other auxiliary loads 28a-28h (such as a ballast pump or a blower motor) that require only a single frequency and voltage power input (as opposed to the variable frequency and voltage input required by the motors 18a-18b) are typically coupled either directly to the single frequency, single voltage power distribution bus 14 or via a step down transformer 30a-30b. 
The high power VFDs 24a-24b required to operate the variable speed motors and, thus, the speed of the propeller, are often expensive and increase the power consumption and overall costs of the conventional power distribution system 10.
Certain older conventional oil tankers, such as the T2-SE-A1 tanker built in circa 1940 by the Sun Shipbuilding Company, employed a steam turbine driven alternator to power and control a 2300 volt 3 phase synchronous/induction electric motor that was directly coupled to the tanker's propeller shaft. A separate 75 kw 110 volt dc generator was required to provide a 110-volt dc excitation to the motor's rotor for synchronous mode operation. This motor normally operated in synchronous mode when the motor was started and running at a non-idle speed in step with the turbine driven alternator. However, to start and run up this motor, it was necessary to place the motor in induction mode. This was achieved by short-circuiting the motor's rotor slip rings, which normally carried the 110-volt dc excitation current to the motor's rotor windings. As soon as the motor got up to speed, which was observed by ammeters falling back, the short circuit was removed and the 110-volt dc excitation current applied to the motor's rotor slip rings to transition to synchronous mode. A person at the motor controls carried out this operation manually. Auxiliary power for other loads on this conventional tanker was provided by a separate steam turbine driven 400 kw, 440 volt 3 phase alternator. Thus, this older conventional oil tanker required separate power distribution busses for the main propeller motor and other ship loads.
Therefore, there is a need for a common power distribution bus that overcomes the problems noted above and others previously experienced for supplying power and controlling a variable speed motor to operate a propeller shaft or other variable speed load while avoiding the inefficiencies and costs of using variable frequency converters between the prime movers and the loads.